As many as 18,000 nurses in the midst of contract negotiations have walked off the job and are picketing in front of Kaiser Permanente facilities in Northern California to express their concerns about patient-care standards and Ebola safeguards.

With nurses holding red and yellow “strike for health and safety” picket signs, the two-day walkout started Tuesday morning and is to last until 7 a.m. Thursday. It will affect at least 21 Kaiser hospitals and 35 clinics.